Prestressed concrete pipe is very commonly used to carry large quantities of bulk liquid, such as potable water, under pressures of typically 150 psig to 300 psig. In a typical example of prestressed concrete conduit, round concrete pipe having a wall thickness of between about 10 inches and about 20 inches is tightly wrapped with prestressing wire having a thickness between about 0.16 inches and 0.35 inches. The prestressing wire is typically then covered with mortar to minimize corrosion of the wire. Such prestressed concrete pipes can conveniently be used below ground.
A serious problem regarding the use of prestressed concrete pipe is the danger of the pipe exploding after a portion of the prestressing wire fails for one reason or another. In such an explosion, it is not unusual for large chunks of concrete to be thrown many hundreds of feet. Such failures are, therefore, very dangerous.
The testing of prestressed concrete pipe is difficult to do in a nondestructive manner. This is because the prestressing wire is thoroughly covered with mortar. Until recently, there has been no accurate method of non-destructive testing of prestressed concrete pipe.
The Washington Suburban Sanitary Commission has attempted to provide a non-destructing method for testing prestressed concrete pipe. The Washington Suburban Sanitary Commission method is set forth in U.S. Pat. No. 5,540,096, the contents of which are incorporated herein by reference. In the Washington Suburban Sanitary Commission method, sound waves are imparted to one side of the concrete pipe and sensed by a single acoustic sensor disposed immediately adjacent the inducing site. One or more parameters of the acoustic wave detected by the acoustic sensor is then correlated with standards to identify discontinuities.
The problem with this method is that it detects discontinuities only at a single point proximate to the acoustic wave induction site. The method is not capable of detecting discontinuities away from the acoustic wave induction site. Accordingly, the method is exceedingly time-consuming and expensive because literally hundreds, if not thousands, of readings have to be taken to cover the entire surface of even a 20-foot section of prestressed concrete pipe.
Accordingly, there is a need for a method whereby prestressed concrete pipe can be simply, inexpensively and quickly tested.